Import latest mdoc(7) fixes of mine.

Fold -xwidth functionality into -width. .Bl now tests whether
string immediately following a leading dot starts with a valid
macro name.

Added similar functionality to the -column list's column width
specifiers. For example, the following now works as expected:

.Bl -column ".Va hw.crusoe.percentage" ".Vt integer" "Changeable"
.It Sy "Name\tType\tChangeable\tDescription"
.It Va hw.crusoe.longrun Ta Vt integer Ta yes Ta "LongRun mode:"
.It Ta Ta Ta "0: minimum frequency mode"
.It Ta Ta Ta "1: power-saving mode"
.It Ta Ta Ta "2: performance mode"
.It Ta Ta Ta "3: maximum frequency mode"
.It Va hw.crusoe.frequency Ta Vt integer Ta no Ta "Current frequency (MHz)."
.It Va hw.crusoe.voltage Ta Vt integer Ta no Ta "Current voltage (mV)."
.It Va hw.crusoe.percentage Ta Vt integer Ta no Ta "Processing performance (%)."
.El

This is especially useful for groff devices with variable width
fonts, like -Tps or -TX100.